DESCRIPTION: If composite resins are to be considered a viable alternative to Dental amalgam, the durability of the dentin/adhesive (d/a) bond must be improved. Under in vivo conditions, this bond can be the first defense against substances that penetrate and ultimately undermine the composite restoration. The durability of the d/a bond is directly related to the quality of the HL but measuring quality has been a formidable problem. To date, the majority of our techniques have provided an indirect assessment, e.g., testing methods that measure fracture resistance of bulk d/a specimens pre- and post-water immersion are not sensitive enough to identify interfacial defects where aqueous degradation begins. Our recent in situ characterization of the molecular structure and micro-mechanics of the hybrid layer has indicated a serious limitation-physical separation of the BisGMA/HEMA adhesive upon mixing with water in the demineralized dentin. The critical dimethacrylate (BisGMA), the component contributing most to the cross-linked polymeric adhesive, infiltrated a fraction of the wet, demineralized dentin. The overall hypothesis of this work is that in the presence of moist clinically relevant dentin surfaces, methacrylate-based adhesives formulated to include water-compatible components will provide decreased phase separation and thus, enhanced structural integrity and increased durability of the hybrid layer as compared to state-of-the-art commercial adhesives. The Specific Aims will test the hypotheses: 1) cationic surfactants, water compatible photoinitiators and water-compatible cross-linking dimethacrylate co-monomers in methacrylate-based adhesive formulations will decrease phase separation that occurs in water and act synergistically to reduce the detrimental effects associated with such phase separation; 2) these adhesives formulated with water-compatible components will provide enhanced dentin infiltration and be miscible with wet demineralized matrices of healthy and caries-affected dentin, assuming suitable viscosity; 3) the module of elasticity at the interface of healthy dentin and caries-affected dentin with methacrylate-based adhesives exhibiting reduced phase separation and water compatible characteristics will be stable upon exposure to aqueous environments and that the interfacial elastic moduli will not lead to stress concentration that will result in failure initiation at the interface during loading; 4) that methacrylate-based adhesives exhibiting reduced phase separation, enhanced infiltration, improved miscibility and structural integrity in the presence of wet demineralized healthy dentin matrices will provide strong, durable d/a bonds as compared to state-of-the-art commercial adhesives.